This invention generally relates to antioxidant compositions, and more particularly, to compositions especially suited for use with irradiated polyolefins and highly filled plastics, such as fire-retardant plastics.
Olefin polymers are particularly subject to severe deterioration from the oxidative action of air at elevated temperatures. They are also vulnerable to degradation by the action of ionizing radiation when large amounts of such radiation are deposited onto the polymer, e.g., amounts of 5 .times. 10.sup.10 ergs per gram and higher. Oxidative deterioration at elevated temperatures, i.e., temperatures above 50.degree. C, is manifested by surface crazing, cracking, and ultimately complete embrittlement.
Since large amounts of antioxidants, in the order of greater than 5%, result in increased cost and decrease in plastic strength, it is desirable to obtain an antioxidant system which will be effective at elevated temperatures over long periods of time in relatively low amounts.
A highly efficient antioxidant system is particularly desirable for irradiated plastics. Often, during irradiation, much of the antioxidant is consumed or rendered incapable of functioning as an antioxidant. It is therefore necessary that an antioxidant system be employed for plastics which are to be irradiated which will be effective and survive radiation when used in small quantities so as to prevent oxidation even after being subjected to irradiation.
Also, it is very desirable to have an antioxidant of high efficiency for highly filled plastics. High amounts of filler are often required in the formulation of fire-retardant plastics. It is necessary to incorporate a large amount of filler into the plastic to provide fire retardant properties. This generally results in a more permeable plastic composition which will more readily undergo oxidative degradation. Additionally, a large quantity of filler results in a decrease in plastic strength, so it is advantageous to have an efficient antioxidant which is effective in small amounts in the order of 5% or less of the plastic composition.
While the use of hydroxy aromatic compounds containing sulfur as an antioxidant is known, the use of phosphonates as an antioxidant has been known only when they contain a hydroxy aromatic compound. I have found that by combining a phosphonate compound with a hydroxy aromatic compound containing sulfur, a more efficient antioxidant system is obtained.
Thus, it is an object of this invention to provide an antioxidant composition suitable for use with various polymers sensitive to oxidation and which is more efficient at elevated temperatures than the antioxidants of the prior art.
It is a further object of this invention to provide an antioxidant composition which is especially suitable for use with irradiated polyolefins.
It is another object of this invention to provide a antioxidant composition particularly suited for use in fire-retardant plastic systems which have a large percentage of filler.